This invention relates to a control apparatus for an internal combustion engine which controls the timing of some feature of engine operation, such as the ignition or fuel injection timing. More particularly, it relates to a control apparatus which can prevent the engine timing from being affected by errors in detection of the crankshaft position of the engine.
An internal combustion engine is generally equipped with a reference position sensor which senses each time a piston of any cylinder of the engine is at a predetermined reference position. The time at which ignition or fuel injection is to take place in each cylinder is measured from these reference positions.
A reference position sensor is typically a device which senses rotation of some member which rotates in synchrony with the engine, such as the crankshaft or camshaft of the engine or the rotor of a distributor for the engine. Due to the limits of manufacturing precision and the inherent limitations on the sensing ability of reference position sensors, the reference positions can only be sensed with an accuracy of .+-.2.degree. of crankshaft rotation. Since the engine timing, such as the ignition timing, is measured from the occurrence of the reference positions as sensed by the reference position sensor, the engine timing may be off by as much as 2.degree. from the correct timing due to sensing error. For example, if the correct ignition timing is 10.degree. BTDC (before top dead center) and the sensing of the reference positions is off by 2.degree., the ignition may actually take place as early as 12.degree. BTDC or as late as 8.degree. BTDC.
When the ignition timing of an engine is the ideal value, the output torque of the engine is a maximum. However, when the ignition timing is only 1.degree. different from the ideal value, the output torque falls by about 1 horsepower. Accordingly, an error component of .+-.2.degree. in the engine timing is far from negligible, and can produce a significant decrease in horsepower. Furthermore, if the ignition timing deviates from the ideal value and an excessive advance angle or abnormal lag angle develops, problems such as engine knocking or insufficient engine output may develop.